The present invention pertains generally to devices and methods for processing multi-species plasmas. More particularly, the present invention pertains to devices and methods for controlling the orbits of particular ions in a plasma by manipulating crossed electric and magnetic fields (Exc3x97B). The present invention is particularly, but not exclusively, useful for tuning an a.c. voltage component of the electric field, in crossed electric and magnetic fields; to control the orbits of ions having a particular mass/charge ratio; and to thereby separate these ions from a multi-species plasma in a predictable way.
A plasma mass filter for separating ions of a multi-species plasma has been disclosed and claimed in U.S. Pat. No. 6,096,220 which issued to Ohkawa (hereinafter the Ohkawa Patent), and which is assigned to the same assignee as the present invention. To the extent it is applicable, the Ohkawa Patent is incorporated herein by reference, in its entirety. In brief, the Ohkawa Patent discloses a plasma mass filter which includes a cylindrical chamber that is configured with axially oriented, crossed electric and magnetic fields (Exc3x97B). More specifically, the electric field, E, has a positive value wherein the voltage at the center (Vctr) is positive and decreases to zero at the wall of the chamber. Further, the electric field (E) has a parabolic voltage distribution radially and the magnetic field (B) is constant axially. Thus, E and B are established to set a cut-off mass, Mc, which is defined as:
Mc=zea2(B)2/8Vctr
where xe2x80x9caxe2x80x9d is the distance between the axis and the wall of the chamber and xe2x80x9cexe2x80x9d is the elementary charge, and xe2x80x9czxe2x80x9d is the charge number of the ion.
In the operation of the plasma mass filter disclosed in the Ohkawa Patent, the crossed electric and magnetic fields (Exc3x97B) place ions on either xe2x80x9cunconfinedxe2x80x9d or xe2x80x9cconfinedxe2x80x9d orbits, depending on the relative values of the mass/charge ratio of the ion xe2x80x9cm,xe2x80x9d and the cut-off mass Mc, as it is established for the filter. Specifically, when xe2x80x9cmxe2x80x9d is greater than Mc, the ion will be placed on an unconfined orbit. The result then is that the heavy ion, (i.e. m greater than Mc), is ejected from the axis on its unconfined orbit and into collision with the wall of the chamber. On the other hand, in these crossed electric and magnetic fields, when an ion has a mass/charge ratio xe2x80x9cmxe2x80x9d that is less than Mc, the plasma mass filter causes the light ion (i.e. m less than Mc) to have a confined orbit. In this latter case, the result is that the light ion will exit the chamber on its confined orbit. The situation changes, however, if the electric field has an a.c. voltage component.
Consider crossed electric and magnetic fields (Exc3x97B) wherein the electric field has both a d.c. voltage component (∇"PHgr"0) and an a.c. voltage component (∇"PHgr"1). A charged particle with a charge/mass ratio xe2x80x9cmxe2x80x9d (i.e. an ion) will have a cyclotron frequency in these crossed electric and magnetic fields which can be expressed as xcexa9=zeB/m, wherein xe2x80x9cexe2x80x9d is the elementary charge of an electron and xe2x80x9czxe2x80x9d is the charge number. Further, a derivation of the equations of motion for ions in a crossed electric and magnetic field, without collisions, yields an expression in the form of a Hill""s equation; namely
d2/dt2s+[xcexa9/4xe2x88x92xcex]s=0.
In this case:
xcex=2eV(t)/ma2
where V(t) is the applied voltage, as a function of time, and xe2x80x9caxe2x80x9d is the distance between the axis and the wall of the chamber. If xcex is sinusoidal, with a frequency, xcfx89; namely
xcex=xcex0+xcex1 cos xcfx89t
the Hill""s equation shown above is transformed into the form of a Mathieu""s equation; namely
[xc2xc]d2/dt2s=[xcex1xe2x88x924xcex2 cos 2xcfx84]s=0
where
xcfx84=xcfx89t/2
xcex1=[xcexa92/4xe2x88x92xcex0]/xcfx892
xcex2=xcex1/[4xcfx892].
For small values of xcex2 the following expressions will define boundaries that differentiate between operational regimes for confined and unconfined orbits. These expressions are:
4xcex10=xe2x88x9225xcex22+257xcex24
4xcex11=1xc2x18xcex2xe2x88x928xcex22
4xcex12=4+80/3 xcex22
The consequence of the above is that when the electric field, E, of crossed electric and magnetic fields is provided with an a.c. voltage component (∇"PHgr"1) the a.c. voltage component can be tuned to place selected ions on an unconfined orbit. This will be so, even though the ions would have otherwise passed through the chamber on confined orbits in the absence of an a.c. voltage component. Further, due to the mass dependence of the above equations, ions of a predetermined mass/charge ratio xe2x80x9cmxe2x80x9d can be selectively targeted for the change from confined orbits to unconfined orbits.
An example of a desirable consequence that can result from the above disclosed phenomenon is provided by the element Strontium (Sr). It happens that the doubly ionized ion species of this element, Sr++90, has the equivalent mass number of 45 (i.e. m=45). With this in mind, consider a plasma mass filter that has been configured with crossed electric and magnetic fields (Exc3x97B) having an established cut-off mass, Mc=75, but with no a.c. voltage component (∇"PHgr"1) for the electric field. Under these circumstances (i.e. m less than Mc) the Sr++90 (with m=45) will be placed on confined orbits and allowed to exit the filter. This, however, may be an undesirable result. Thus, in accordance with the mathematical calculations discussed above, an a.c. voltage component (∇"PHgr"1) that is introduced into the electric field can be tuned to take out the Sr++90 by placing these ions on unconfined orbits. In this particular example, it can be mathematically shown that the Sr++90 will be taken out of the plasma (i.e. ejected into the wall of the plasma chamber) if the a.c. voltage component (∇"PHgr"1) is tuned with an r.f. frequency xcfx89=0.63.xcexa9.
In light of the above, it is an object of the present invention to provide a band gap plasma filter that can effectively change the characteristic orbit of selected ions from confined to unconfined orbits. Yet another object of the present invention is to provide a band gap plasma filter with crossed electric and magnetic fields that place selected ions of a multi-species plasma on unconfined orbits, while ions of higher and lower mass/charge ratios can be placed on confined orbits. Still another object of the present invention is to provide a band gap plasma filter that is easy to manufacture, is simple to use, and is cost effective.
A band gap plasma filter for selectively controlling ions of a multi-species plasma having a predetermined mass/charge ratio (m1) includes a plasma chamber and a means for generating crossed electric and magnetic fields (Exc3x97B) in the chamber. More specifically, the chamber itself is hollow and is substantially cylindrical-shaped. As such, the chamber defines an axis and is surrounded by a wall.
In order to generate the crossed electric and magnetic fields (Exc3x97B) in the chamber, magnetic coils are mounted on the chamber wall, and electrodes are positioned at the end(s) of the chamber. Specifically, the magnetic coils establish a substantially uniform magnetic field (B) that is oriented along the axis of the chamber. The electrodes, however, create an electric field (E) with an orientation that is in a substantially radial direction relative to the axis. Importantly, as envisioned for the present invention, the electric field has the capability of having both a d.c. voltage component (∇"PHgr"0) and an a.c. voltage component (∇"PHgr"1) (i.e. E=∇("PHgr"0+"PHgr"1). Specifically, the d.c. component of the voltage (∇"PHgr"0) is characterized by a constant positive voltage, Vctr, along the axis of the chamber, and has a parabolic dependence on radius with a substantially zero voltage at the wall of the chamber. On the other hand, the a.c. component of the voltage (∇"PHgr"1) will be sinusoidal and is tunable with an r.f. frequency, xcfx89.
In the operation of the band gap filter of the present invention, the d.c. voltage component (∇"PHgr"0) of the electric field, E, can be fixed as discussed above, to establish a cut-off mass, Mc=zea2(B)2/8Vctr. When m1 less than Mc, and the a.c. voltage component (∇"PHgr"1) of the electric field, E, is substantially zero, the d.c. voltage component (∇"PHgr"0) will place the ions m1 on confined orbits in the chamber. In this case the band gap filter of the present invention operates substantially the same as the Plasma Mass Filter disclosed and claimed in the Ohkawa Patent. Accordingly, the ions m1 will pass through the chamber on their confined orbits. The introduction of a predetermined a.c. voltage component (∇"PHgr"1) into the electric field, E, however, will change this.
In addition to the components which generate the crossed electric and magnetic fields (Exc3x97B), the band gap filter of the present invention includes a tuner for tuning the amplitude and frequency, xcfx89, of the a.c. component (∇"PHgr"1) of the voltage. Specifically, for the example discussed above wherein m1 less than Mc, the a.c. voltage component (∇"PHgr"1) can be tuned so that the ions m1 will be placed on unconfined orbits in the chamber, rather than being placed on the confined orbits they would otherwise follow when there is no a.c. voltage component (∇"PHgr"1). More specifically, this is possible by selectively tuning the a.c. voltage component (∇"PHgr"1) with a radio frequency, xcfx89, according to values of xcex1 and xcex2, wherein
xcex1=[xcexa92/4xe2x88x92xcex0]/xcfx892
xcex2=xcex1/[4xcfx892].
The consequence of the above is that when placed on unconfined orbits, the ions m1 will move away from the axis of the chamber and be ejected into collision with the wall. Thus, rather than passing through the chamber on confined orbits, the ions m1 can be selectively prevented from passing through the chamber. For a multi-species plasma that includes both the ions m1, as well as ions of a second mass/charge ratio (m2), the band gap filter of the present invention can selectively prevent these ions (either m1, or m2, or both) from passing through the chamber.